Fuel cells allow a fuel capable of producing a proton (for example, hydrogen) to be electrochemically reacted with an oxidizing agent containing oxygen (for example, air), to generate electric power.
Cathode electrodes of fuel cells are configured by supporting precious metal catalyst particles having a small particle size on electrically conductive porous fine powders such as carbon fine powders. Upon operation of fuel cells, concomitantly with electron transfer from the electrically conductive fine powders to the catalyst particles, protons move from the electrolyte membrane to the catalyst particles, and thus a catalytic reaction of the electrons, gaseous oxygen and the protons proceed on the catalyst particle surfaces to produce water.
The reaction center where the catalytic reaction occurs is generally referred to as three-phase interface. The area of this three-phase interface is an effective area of the catalyst particles with which the protons can be efficiently in contact, and as this area becomes greater, availability of the catalyst is improved, leading to improvement of the cell performances.
For manufacturing electrode for fuel cells, a method has been proposed in which a polymer electrolyte is mixed by stirring with electrically conductive fine powders supporting catalyst particles to cover the catalyst particle surface with the polymer electrolyte (see, for example, Patent Document 1, paragraph No. 0002).
However, in such a catalyst layer formed by mixing the electrically conductive fine powders supporting the catalyst particles with the polymer electrolyte material by stirring, the catalyst surface is embedded in the polymer electrolyte material, and thus problems based on a small area of the three-phase interface do occur.
In this respect, in order to expose the catalyst particles on the uppermost surface, a method of forming an electrode has been proposed which comprises forming a porous electrode layer supporting catalyst fine particles, and thereafter applying a dispersion liquid of a polymer electrolyte on the electrode layer (see, for example, Patent Document 2, paragraph Nos. 0008-0011).
As the electrolyte membrane for fuel cells and the electrolyte layer in the electrode, perfluorosulfonic acid based polymer electrolyte typified by Nafion® (manufactured by Du Pont Kabushiki Kaisha, trade name) has been generally used.
Since these polymer electrolyte materials have a great particle size in the dispersion solvent, they do not fill small air spaces contained in the porous electrode layer. Thus, the electrolyte material does not reach the vicinity of the catalyst fine particles, and as a result, the problems based on a small area of the three-phase interface have not been solved yet.
As additional other polymer electrolyte materials, hydrocarbon polymer based sulfonic acid electrolytes, and polysiloxane based sulfonic acid electrolytes have been reported (see, for example, Patent Documents 3 and 4). Alternatively, electrolyte materials using inorganic oxide particles modified with an ionic functional group such as a sulfonic acid group on the surface have been also proposed (see, for example, Patent Documents 2 and 5).
Since these polymer electrolyte materials have a great particle size in a dispersion solvent, they do not fill small air spaces contained in the porous electrode layer. Thus, the electrolyte material does not reach the vicinity of the catalyst fine particles, and as a result, the problems based on a small area of the three-phase interface have still not been solved.
Accordingly, there have been problems of a small effective area of the catalyst particles as a result of the small area of the three-phase interface since the contact area of the catalyst fine particles with the electrolyte material is not sufficient.